1. Field of the Invention
The present invention relates to a method for preventing density unevenness in an image with respect to a feeding direction of a recording material, and an image forming apparatus having a function for preventing density unevenness.
2. Description of the Prior Arts
As an image forming apparatus for recording an image on a recording material, there are an inkjet printer with an inkjet head and a thermal printer with a thermal head. In these printers, the recording material is fed by a feed roller pairs to be driven by a motor.
In the above-mentioned image forming apparatus, the feeding speed of the recording material may be uneven mainly due to an eccentric or distorted rotation shaft of the motor, and this may cause density unevenness in the image along the feeding direction of the recording material. The density unevenness occurs periodically. Therefore, the parts for the feed transport driving system, from the motor to the feed roller pairs, are made and assembled as accurate as possible in the prior art image forming apparatus.
In the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 5-220946, the density of an image pattern recorded at a predetermined density is measured, and then a correction data is created for correcting the image signal to be output to each recording element of the recording head according to the detected density, so that the image is recorded based on the image signal corrected by the correction data. In addition, Japanese Patent Laid-Open Publication No. 6-54150 discloses the jitterless image forming apparatus in which the driving unevenness of the photoreceptor for feeding the recording paper is detected and the writing timing of the optical writer is altered according to the detected driving unevenness signal.
In the prior art image forming apparatus, although the parts accuracy of the feed driving system is enhanced in order to eliminate the density unevenness due to the unevenness in the feeding speed, there arises a problem that the parts cost is increased. In addition, the above-mentioned image forming apparatuses deter the occurrence of the density unevenness by controlling the driving timing of the recording head and the optical writer; however, since the output from the recording head and the optical writer are greatly influenced by incidental parameters such as environmental temperature and humidity around the image forming apparatus, it is difficult to prevent the density unevenness surely.